The European Union's executive is due to approve a further 3 billion euros ($3.7 billion) in funding for Syrian refugees living in Turkey, EU officials said, before a meeting with President Tayyip Erdoğan later this month.

Top EU officials will meet Erdoğan on March 26 in the Bulgarian city of Varna despite misgivings among many on the European side.

The bloc's top migration official Dimitris Avramopoulos will announce on Wednesday that the European Commission proposes the extra funding on projects benefiting Syrian refugees in Turkey, the sources told Reuters.

Turkey has accepted 3.5 million refugees from Syria, and the EU is already spending a first 3 billion euro instalment to help them.

Brussels agreed to pay to help host migrants on the Turkish soil in exchange for Ankara preventing more from trying to cross the Aegean to Greece.

The EU will also release in April what the sources said would be a "critical" report on Turkey's accession bid.